1. Field of the Invention
The present invention relates generally to gas turbine engine axisymmetric vectoring exhaust nozzles and, more particularly, to a failsafe system for the actuating system of such engines.
2. Discussion of the Background Art
Military aircraft designers and engineers constantly strive to increase the maneuverability of the aircraft, both for air to air combat missions and complicated ground attack missions. They have developed thrust vectoring nozzles, which turn or vector the exhaust flow and thrust of the gas turbine engine powering the aircraft, to replace or augment the use of conventional aerodynamic surfaces such as flaps and ailerons. One newly developed thrust vectoring nozzle is an axisymmetric vectoring exhaust nozzle disclosed in U.S. Pat. No. 4,994,660, by Hauer, herein incorporated by reference. The axisymmetric vectoring exhaust nozzle provides a means for vectoring the thrust of an axisymmetric convergent/divergent nozzle by universally pivoting the divergent flaps of the nozzle in an asymmetric fashion or, in other words, pivoting the divergent flaps in radial and tangential directions with respect to the unvectored nozzle centerline. The flaps are pivoted by a vectoring ring which can be axially translated and gimballed or tilted about its horizontal and vertical axis (essentially have its attitude adjusted) through a limited range.
The axisymmetric vectoring exhaust nozzle, as well as more conventional gas turbine engine exhaust nozzles, include primary and secondary exhaust flaps arranged for defining a variable area convergent-divergent exhaust nozzle. The exhaust nozzle is generally axisymmetric or annular and exhaust flow is confined by the primary or convergent flaps and, secondary or divergent flaps being positioned circumferentially adjacent to each other, respectively. The divergent flaps, for example, have a forward end defining a throat of minimum flow area and an aft end having a larger flow area for defining a diverging nozzle extending downstream from the throat. The divergent flaps are variable, which means that the spacing between the divergent flaps as they are moved from a smaller radius position to a larger radius position must necessarily increase. Accordingly, conventional exhaust nozzle seals are suitably secured between adjacent divergent flaps to confine the exhaust flow and prevent leakage of exhaust flow between the divergent flaps.
Vectoring nozzles and, in particular, axisymmetric vectoring exhaust nozzles of the type disclosed in the Hauer reference, provide positionable divergent flaps. These divergent flaps are positionable not only symmetrically relative to a longitudinal centerline of the exhaust nozzle, but may also be positionable asymmetrically relative thereto for obtaining pitch and yaw vectoring. An exemplary thrust vectoring nozzle uses three vectoring actuators to translate and tilt a vectoring ring which in turn forces the divergent flaps in predetermined positions. The vectoring ring tilt angle and tilt direction establish the nozzle's vector angle and vector direction, respectively. Axial translation of the vectoring ring establishes the exit area (often referred to as A9) for a given throat area (often referred to as A8).
Modern multi-mission aircraft application employ engines, such as the GE F110 engine, with convergent/divergent nozzles to meet operational requirements. Convergent/divergent nozzles have, in serial flow relationship, a convergent section, a throat, and a divergent section. Characteristically, these nozzles employ variable area means at both the nozzle throat and at the nozzle exit. This provides a means to maintain a desired exit to throat area ratio, which in turn allows efficient control over the operation of the nozzle. The operation of the nozzle is designed to provide a nozzle exit/throat area (A9/A8) schedule which is optimized for the design cycle of the engine and ideally should provide efficient control at both low subsonic and high supersonic flight conditions. These types of nozzles typically use pneumatic or hydraulic actuators to provide the variable operation. Typically, the exit and throat areas are mechanically coupled to each other in such a manner as to create an area ratio (A9/A8) schedule which is a function of nozzle throat area (A8). The area ratio schedule is typically predetermined to provide efficient engine operation across a wide range of engine conditions but typically optimum performance at specific engine conditions is compromised somewhat in order to provide adequate efficiency throughout the range of engine operation. Thrust vectoring nozzles typically have the ability to independently control nozzle exit area and throat area which allows the engine to achieve a higher level of performance across a wide range of engine operating conditions.
During engine and aircraft operation it is possible for the hydraulic actuating system for the nozzle to fail in any one or more of several modes due to a component malfunction or damage such as due to combat. The failure may be due to a mechanical or control system malfunction which is typically detected by a flight control computer and/or a vector electronic control used for a thrust vectoring nozzle. The nozzle actuating system and nozzle is therefore typically provided with a hydraulic failsafe position using actuating ring actuators to fully retract and in the case of a vectoring ring to set the nozzle in a fixed unvectored position so that thrust of the engine is not vectored. These vectoring actuators are also used to control A9. However the resulting nozzle geometry has a very large area ratio (A9/A8) which impedes opening A8 and therefore augmentor operation and is not aerodynamically optimal. Such a failsafe system is unacceptable. The large area ratio also can cause flow separation of the exhaust plume inside the divergent section of the nozzle. Intermittent separation and reattachment of the flow, particularly in an asymmetric fashion with respect to the engine centerline, could result in an inadvertent vector force. Fully opening the divergent portion of the nozzle results in vastly different nozzle kinematics and opening the throat of the nozzle at this high area ratio could severely damage the nozzle. The inability to open the nozzle throat prevents nominal operation of the engine at ground idle conditions and in the afterburner mode, which could cause the operation of the aircraft to deviate from the norm.
These shortcomings are some of the reasons that there exists a need for a means to provide failsafe mechanism that can rapidly configure the nozzle in a safe operating mode in case of certain types of hydraulic system failure. The failsafe system should operate with a minimal adverse effect on the overall operability of the aircraft and its engine, particularly during combat.